1. Field of the Invention
The present invention relates to a technique for increasing the rain margin of a TDMA satellite communication system and, more particularly, to a technique which permits communication between ground stations of a TDMA satellite communication system via the satellite where one or more ground stations are experiencing a fade condition above the power margin.
2. Description of the Prior Art
The current trend in communication satellites appears to be increasingly toward the use of the 12/14 GHz and higher frequency bands and the uses of digital modulation formats with Time Division Multiple Access (TDMA) techniques. The former provides freedom from existing 4/6 GHz terrestrial interference and also provides higher antenna gain and narrower beams for a given size aperture, while digital transmission in conjunction with TDMA provides for more efficient utilization of the available satellite system resources.
A major drawback associated with 12/14 GHz systems is the signal attenuation associated with rainfall. In general, attenuation at these frequencies is an increasing function of rain rate, with the result that, for example, over a large portion of the United States, significant power margin must be provided to prevent excessive outage due to rain fades.
A typical prior art technique for overcoming rain fades is disclosed in an article "The Future of Commercial Satellite Telecommunications" by W. White et al in Datamation, July 1978 at pp. 94-102 which discloses at pp. 98-99 that it may be possible to overcome rain attenuation in satellite systems by transmitting the same burst several times. The ground station in the momentary rain zone can add the multiple signals for the same burst together to reconstruct the original signal.
Other standard techniques which might be employed to provide rain margin include (1) increasing the radiated power of the satellite and earth stations, (2) improving the noise figure of the receivers, (3) installing larger ground station antennas, and (4) providing site diversity. Unfortunately, these techniques (1)-(4) are costly in that permanently dedicated system resources are used only infrequently, i.e., when it rains. Therefore, the system has been tremendously overdesigned for the clear air conditions which might exist more than 99.9 percent of the time at any particular ground location if, for example, 15 or 20 dB rain margin is required to achieve the desired rain outage.
The problem remaining in the prior art is to provide method and apparatus which can increase the rain margin of a satellite communication system by as much as, for example, 10 dB without requiring additional system resources which are only infrequently called upon for use.